Oxidatively curing coating agents such as paints and clear coats, as a rule, contain resin systems as binders which crosslink by absorbing atmospheric oxygen. In general, this crosslinking process is very slow such that the drying and curing of such coating agents requires, or can require, a great deal of time. To accelerate this curing process siccatives, also referred to as “driers” are employed. According to DIN EN 971-1 (1996) these siccatives are most frequently metal salts of organic acids which are soluble in organic solvents and binders. The most commonly employed drying agents are salts of transition metals which occur in at least two non-zero oxidation states, in particular salts of cobalt and manganese (primary siccatives). These are also utilized in combination with further metal salts which are not siccatives themselves which, however, as secondary siccatives exhibit synergistic effects or ameliorate or compensate the most unfavorable properties of the primary siccatives. Preferred organic acids are, for example, naphthenic acid, isooctanic acid or linseed fatty acid. The mass fraction of the siccatives in oxidatively drying coating agents is conventionally between 1% and 3% referred to the mass of the resin fraction in the binder, corresponding to a mass fraction of 0.1% to 0.3% of the metals.
Patent application WO 2001/046294 describes a coating agent which comprises an alkyd resin, a siccative (preferably mangano-octoate), activated with a complexing agent such as 2,2′-bipyridyl or 1,10-phenanthroline, and a polythiol. This type of coating agent, due to the manganese siccative, leads to undesirable discolorations with the coating agent developing a brownish color. In addition, 2,2′-bipyridyl or 1,10-phenanthroline in connection with iron ions, which, for example, derive from a metallic substrate, tend toward the formation of strongly colored complexes.
Heavy metals, such as manganese or cobalt, frequently utilized in siccatives, not only entail disadvantages with respect to their color-changing properties, but they are also of concern with respect to their environmental compatibility.